I just experienced this for the first time this week. Several files that I
copy and update every month have suddenly become inaccessible. I worked in
them and saved with no problem. I tried to open them a day later and I first
get a message "Document may be read only or encrypted" then when I take an
OK, I get "Cannot access filename".

I went to the prior month's files and can open them normally. But if I go
back a few months (and these have not been touched for several weeks) and try
to open that month's version of the files I get the error messages. The only
commonality I can see with the files I'm having difficulty with is that they
are all ones I copy from a prior version and modify. I don't think there is
an issue with authority because I had our system administrator try to open
the files as well.

I am working in Office 2003 on XP and have processed all updates
automatically. I know there were some updates recently to Office and I'm
wondering if these may have an impact.